WHEEL WELL FIRE DETECTION -COMPONENT LOCATIONS

Wheel Wells
Each wheel well has two wheel well fire detectors.
Main Equipment Center
These are the components in the main equipment center that have an interface with the wheel well fire detection system:
*
Left DLODS control card

*
Left and Right ASG cards (two each)

*
Right DLODS control card.

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WHEEL WELL FIRE DETECTION -WHEEL WELL FIRE DETECTORS -INTRODUCTION

Purpose
Wheel well fire detectors monitor the main gear wheel wells for tire fires.
Physical Description
The wheel well fire detector has two electrical elements with connectors at each end. Each detector element has an outer conductor around an inner conductor. The material between these conductors is an electrical insulator at temperatures less than 575 F (302 C).
Location
A pair of detectors are at the top of each main gear wheel well. These detectors are above the wheels when the landing gear retracts.
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WHEEL WELL FIRE DETECTION -WHEEL WELL FIRE DETECTORS -INTRODUCTION

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WHEEL WELL FIRE DETECTION -WHEEL WELL FIRE DETECTORS -FUNCTIONAL DESCRIPTION

Functional Description
When a fire occurs, the material between the conductors melts. In the liquid condition this material conducts electricity.
Training Information Point
A clip attaches a pair of adjacent detector elements to structural points in each wheel well. Connectors on each end connect detectors together in series through airplane wiring.
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WHEEL WELL FIRE DETECTION -FUNCTIONAL DESCRIPTION

Fire Detection
The wheel well fire detection system has two detector loops that operate independently.
The left duct leak and overheat detection system (DLODS) control card monitors loop 1. The right (DLODS) control card monitors loop 2. Each card electrically excites its related loop at a specified voltage. The signal from the loop shows a high electrical resistance at temperatures less than the fire set point (575 F, 302 C). At the fire set point or higher, the signal from the loop shows a low electrical resistance.
If the DLODS control cards find low resistance in both loops of a wheel well, the cards send a signal to the AIMS to indicate a fire. A warning message shows on EICAS. The warning electronic unit (WEU) turns on the master warning lights and fire aural warning.
Loop Failure
The DLODS control cards continuously monitor the detector loops for open and short circuits. If a card finds a short circuit in only one loop of a wheel well, the cards send a signal to AIMS to show a short circuit failure of the detector.
If the signal from the loop shows an open circuit, The DLODS control cards send a signal to AIMS to show an open circuit failure of the detector.
A short or open circuit failure of one detector loop causes maintenance messages. A failure of the two loops causes a status message to show.
DLODS Control Card Failure
If a DLODS control card has a failure, a status message on EICAS show the loop monitored by that card as failed. The serviceable card then uses the other loop as the only source of fire detection.
Fire/Overheat Test
A fire/overheat test causes the DLODS control cards to do a test of the wheel well fire detectors. If the cards find an open or a short circuit in the loop, they send a signal to AIMS. EICAS messages report the results of the test. Failure of the two loops causes an advisory message to show.
Training Information Point
The advisory message that shows as a result of the fire/ overheat test only shows for this manual test. If the DLODS control card finds a failure during continuous testing, there is no advisory message.
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